A primary focus and application of the present invention is the field of transmitters and radio frequency (RF) power amplifiers capable of use in wireless telecommunication applications. Continuing pressure on the limited spectrum available for radio communication systems is forcing the development of spectrally-efficient linear modulation schemes. Since the envelopes of a number of these linear modulation schemes fluctuate, these result in the average power delivered to the antenna being significantly lower than the maximum possible power, leading to poor efficiency of the power amplifier. Specifically, in this field, there has been a significant amount of research effort in developing high-power efficient topologies capable of providing useful performance in the ‘back-off’ (linear) region of the power amplifier.
Linear modulation schemes require linear amplification of the modulated signal in order to minimise undesired out-of-band emissions from spectral re-growth. However, the active devices used within a typical RF power amplifier are inherently non-linear by nature. Only when a small portion of the consumed direct current (DC) power is transformed into RF power, can the transfer function of the amplifying device be approximated by a straight line, i.e. as in an ideal linear amplifier. This mode of operation provides a low efficiency of DC to RF power conversion.
Additionally, the emphasis in portable (subscriber) equipment is to increase battery life. To achieve both linearity and efficiency, so called linearisation techniques are used to improve the linearity of the more efficient amplifier classes, for example class ‘AB’, ‘B’ or ‘C’ amplifiers. A number and variety of linearising techniques exist, such as Cartesian Feedback, Feed-forward, and Adaptive Digital Pre-distortion (DPD), which are often used when designing linear transmitters.
In order to increase the bit rate used in transmit uplink communication channels, larger constellation modulation schemes, with an amplitude modulation (AM) component are being investigated and, indeed, becoming required. These modulation schemes, such as sixteen-bit quadrature amplitude modulation (16-QAM), require linear PAs and are associated with high ‘crest’ factors (i.e. a degree of fluctuation) of the modulation envelope waveform. This is in contrast to the previously often-used constant envelope modulation schemes and can result in significant reduction in power efficiency and linearity.
To help overcome such efficiency and linearity issues, for various communications standards, a number of techniques have been proposed.
Referring to FIG. 1, a known output power spectrum mask 100 of WiFi™ enabled devices, showing the output power limits vs bandwidth from the carrier frequency 115 of a WiFi™ transmission, is illustrated. As illustrated, in WiFi™ enabled devices, the output power spectrum 100 requirement limits the output power 105. Linearization techniques, for example using pre-distortion, that may be employed to linearise the transmit signal, are unable to prevent spectral regrowth 110 at low output power levels distal from the carrier frequency 115. Thus, mechanisms to better control spectral regrowth are desired.
FIG. 2 illustrates a block diagram 200 of a known transmitter architecture that uses a digital pre-distortion (DPD) technique. Here, a signal generator 205 generates a DPD digital training signal (Xref) 210 that is routed through the transmitter circuit, converted to analog form in a digital-to-analog converter (DAC) 220 and particularly routed through a power amplifier 225, such that the output signal (XPA) 230 is an amplified analog representation of the DPD digital training signal (Xref) 210. A portion of the output signal (XPA) 230 is routed back to the DPD circuit and converted back to digital form 240 in analog-to-digital converter (ADC) 235, and subsequently compared to the DPD digital training signal (Xref) 210 in a comparison circuit 245. A calibration circuit (engine) 250 determines how the transmitter circuitry, and particularly the power amplifier 225, has affected the DPD digital training signal (Xref) 210 by analyzing the output from the comparison circuit 245 and determining PA nonlinearity (amplitude modulated to amplitude modulated (AM-to-AM) and amplitude modulated to phase modulated (AM-to-PM)) effects. The calibration circuit (engine) 250 then adapts phase and gain components in the DPD compensation circuit 215 that, effectively, pre-distorts the input signal, e.g. DPD digital training signal (Xref) 210, to compensate for the subsequent non-linearity and distortion effects that will be caused to the input signal by the transmitter circuit. In this manner, a linear transmitter signal is output from the power amplifier, with the inherent non-linearity effects that would have been created cancelled out by the DPD applied by the compensation circuit 215.
In order to meet output power spectrum requirement limits, such as those illustrated in FIG. 1, known transmitters selectively enable or disable DPD compensation circuits, for example based on measurement results of the output power spectrum, typically via an output power spectrum density (PSD) measurement of the PA fed back signal. Alternatively, in order to meet output power spectrum requirement limits, such as those illustrated in FIG. 1, known transmitters may just attenuate the whole output signal to reduce the output power across the whole bandwidth of operation in order to meet the spectrum mask.
U.S. Pat. No. 8,446,979 (2013, PMC-Sierra) describes a technique of envelope sharing whereby coefficients of a DPD are adapted to provide a soft-clip type of filter response characteristic versus a hard-clip type filter response characteristic to reduce the Peak-to-Average Power Ratio PAPR of the modulation signal. However, such a technique is best suited to the higher modulation coding scheme versions of communication standards. The book titled ‘RF Power Amplifiers for Wireless Communications’ and authored by Steve. C. Cripps, also describes a mechanism to compensate for predistortion as an input signal is sampled.
Thus, there exists a need for a more efficient and cost effective solution to reduce spectral re-growth control in RF transmitters, particularly for RF transmitters that employ linearization techniques such as DPD.